Noise reduction

Assuming that you're running version 14.4 or later –

to revert to the original photo, click on any previous step in the History panel.

Same way you would change any "slider", set it back to zero; or uncheck the Denoise box; or pick the state in History before noise reduction; or hit the Reset button; or probably a bunch of other ways that I can't think of right now.

Which version of Lightroom do you use? 

Since version 14.4 is non destructive like other settings. In the former versions Denoising creates a seperate .DNG file. You can go back to the corresponding RAW file but all settings tht have done after denoising will be lost in this case.